Keyboard shortcuts for your Mac at boot time

Keep one of these keys pressed at system start:

Command key
Control key
Option key
Shift Key
Caps Lock
fnFunction Key

 

CBoots from a CD/DVD with a system image
DBoots from the first HD partition
NBoots from the network (netboot)
REnforces a screen reset
TBoots in firewire target mode
Boots in safe mode. Disables login items and less important kernel files
⌘VBoots in verbose mode
⌘SBoots in single-user mode. Goes directly into command-line
 Displays possible startup disks
⌘RBoots in recovery mode. Allows to reinstall the Operating System

… and much more: http://support.apple.com/kb/HT1343